Pre-treatment of lignocellulose biomass for conversion to chemicals requires significant residence time, high pressure and high temperature. Liquids must be separated form the treated biomass at those conditions to achieve a high yield and process efficiency. Currently, multiple pieces of equipment are required to achieve this, which are costly in terms of capital and operating cost. Moreover, process efficiency is marginal.
A key component of process efficiency in the pretreatment of lignocellulosic biomass is the ability to wash and squeeze hydrolyzed hemi-cellulose sugars, toxins, inhibitors and/or other extractives from the solid biomass/cellulose fraction. It is difficult to effectively separate solids from liquid under the high heat and pressure required for cellulose pre-treatment.
During solid/fluid separation, the amount of liquid remaining in the solid fraction is dependent on the amount of separating pressure applied, the thickness of the solids cake, and the porosity of the filter. The porosity of the filter is dependent on the number and size of the filter pores. A reduction in pressure, an increase in cake thickness or a decrease in porosity of the filter, will all result in a decrease in the degree of liquid/solid separation and the ultimate degree of dryness of the solid fraction.
For a particular solids cake thickness and filter porosity, maximum separation is achieved at the highest separating pressure possible. For a particular solids cake thickness and separating pressure, maximum separation is dependent solely on the pore size of the filter.
High separating pressures unfortunately require strong filter media, which are able to withstand the separating pressure, making the process difficult and the required equipment very costly. When high separating pressures are required, the thickness of the filter media needs to be increased to withstand those pressures. However, to maintain the same overall porosity as the filter with the thinner filter media, thicker filter media require a larger pore size. This may create a problem, depending on the solids to be retained, since the acceptable pore size of the filter is limited by the size of the fibers and particles in the solids fraction, the clarity of the liquid fraction being limited solely by the pore size of the filter media. Pores that are too large allow a significant amount of suspended particles to collect in the liquid fraction, thereby reducing the liquid/solid separation efficiency.
Over time, filter media tend to plug with suspended solids reducing their production rate, especially at the high pressures required for cellulose pre-treatment. Thus, a backwash flow of liquid is normally required to clear a blockage and restore the production rate. Once a filter becomes plugged, it takes high pressure to backwash the media. This is particularly problematic when working with filter media operating at pressures above 1000 psig with a process that is to be continuous to maximize the production rate and to obtain high cellulose pre-treatment process efficiency. The current equipment required to effectively perform cellulose pre-treatment is both complex and expensive as there is no known equipment available for simultaneously carrying out multiple lignocellulosic biomass pretreatment steps in a single apparatus.
Conventional single, twin, or triple screw extruders do not have the residence time necessary for low energy pre-treatment of biomass, and also do not have useful and efficient solid/fluid separating devices for the pre-treatment of biomass. U.S. Pat. No. 7,347,140 discloses a screw press with a perforated casing. Operating pressures of such a screw press are low, due to the low strength of the perforated casing. U.S. Pat. No. 5,515,776 discloses a worm press and drainage perforations in the press jacket, which increase in cross-sectional area in flow direction of the drained liquid. U.S. Pat. No. 7,357,074 is directed to a screw press with a conical dewatering housing with a plurality of perforations for the drainage of water from bulk solids compressed in the press. Again, a perforated casing or jacket is used. As will be readily understood, the higher the number of perforations in the housing, the lower the pressure resistance of the housing. Moreover, drilling perforations in a housing or press jacket is associated with serious challenges when very small apertures are desired for the separation of fine solids. Thus, an improved dewatering module for a screw press is desired.